As a means for measuring a solute concentration in an aqueous solution, chemical analysis, or instrumental analysis such as atomic absorption spectroscopy, ion chromatography, etc. has been adopted heretofore.
Further as a simpler means, a hydrometer, an optical refraction type gravimeter, a conductivity meter, etc. may be used.
There are many demands on simple measurement of aqueous solutions of acids such as sulfuric acid, hydrochloric acid, acetic acid, etc. but the measurement is extremely difficult. For example, a concentration of sulfuric acid which is an electrolyte of a lead-acid battery is in a close relationship to a charge and discharge state of the lead-acid battery and therefore, it has been strongly desired to develop a simple method for measuring a sulfuric acid concentration. However, such an adequate measurement technique has not been attained yet. And it is general to determine the concentration with a hydrometer, an optical refraction type gravimeter, etc. described above. As a particular method, it is proposed in West German Patent No. 2,254,207 (1973) to determine a sulfuric acid concentration by measuring a water vapor pressure of the upper space beyond a sulfuric acid electrolyte of a lead-acid battery. That is, this method comprises measuring a water vapor pressure in the upper space in a gas-liquid equilibrated state with a sulfuric acid aqueous solution to determine a concentration of the sulfuric acid aqueous solution since the water vapor pressure of the space varies depending upon the sulfuric acid concentration. This proposal has further been advanced and it has been proposed in J. L. Weininger, J. L. Briant., J. Electrochem. Soc., VOL 129, 2409 (1982) to cover a humidity sensor with a porous polypropylene film, directly immersing the covered humidity sensor in a sulfuric acid aqueous solution and measure partial pressure of water vapor diffused through pores of the porous polypropylene film.
Conventional chemical analysis and instrumental analysis described above generally require equipment in a large scale and at the same time, operations are complicated. Even the operation with a hydrometer, an optical refraction type gravimeter and a conductivity meter are complicated and skill is required for measurement. Besides, it is generally inadequate for continuously monitoring a change in concentration of an aqueous solution.
Upon measurement of a concentration of an aqueous solution of acids such as sulfuric acid, etc. described above, in particular, sulfuric acid as an electrolyte in a lead-acid battery, a hydrometer or an optical refraction type gravimeter has an excessively large size so that the meter cannot be inserted into the lead-acid battery, which makes continuous measurement impossible.
According to the method of West German patent supra, when a sulfuric acid concentration changes, it takes a long time until the water vapor pressure of the sulfuric acid aqueous solution and the water vapor pressure of the space reach equilibrium and a response speed is slow. Further, according to the method of Weininger, et al, the porous polypropylene film has a problem in its material, that is, owing to insufficient water repellency, not only water vapor but also the sulfuric acid aqueous solution itself permeate through the film to reach a moisture-sensitive part of the humidity sensor so that it is difficult to properly determine a humidity. In addition, the life of a sensor is extremely short because of chemical corrosion by sulfuric acid.